Figure 3.

K_d values for ptRNA^Gly binding to M1 RNA determined by the spin column assay (8,23). Assay buffers contained 50 mM MES, pH 6.0, 1 M NH₄OAc and indicated concentrations of Sr²⁺ and/or Zn²⁺. (A) Bar diagram of log K_d dependence on Sr²⁺ and/or Zn²⁺ (or Cd²⁺) concentration as indicated below the diagram. Values above bars represent the proportion of ptRNA^Gly that was able to form a complex with M1 RNA at the theoretical endpoint (M1 RNA saturation), normalized to conditions of 40 mM Sr²⁺. Asterisk above the black bar for conditions of 20 mM Zn²⁺ alone: the experimental endpoint could not be reached due to very weak complex formation, which resulted in high errors for K_d determinations; the K_d of ≥20 μM is therefore only an estimate. Individual values are based on average on four independent experiments; errors are indicated by error bars. (B) Hill plot analysis of K_d dependence on Sr²⁺ concentration. f = (minimal K_d)/(observed K_d); the minimal K_d at saturating Sr²⁺ concentrations was determined as 2 nM.